Translators don't have to repeat the "main station". They can separately feed FM quality audio.
Again, they can feed FM quality audio to the translator separately as long as it is a 100% duplication. There are hundreds and hundreds of translators for AM stations that get studio quality audio for their translator. And, of course, daytime AMs with translators can operate 24/7 and the translator does not have to depend on the AM signal at all.
There have even been cases where the AM has technical issues and is off the air but the translator continues on.
FM Translators and Boosters has all the rules for translators.